Self Assessment deadlines
All the Self Assessment dates that matter, from registration through to payments on account.
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The Self Assessment dates that matter
Four dates cover almost everything. Register for Self Assessment by 5 October after the end of the tax year you first need to file for. Paper returns are due 31 October. Online returns and the balancing payment are due 31 January. And if you make payments on account, the second one falls due 31 July.
The tax year itself runs 6 April to 5 April — so the return you file by 31 January 2027 covers the year ended 5 April 2026.
Payments on account, explained
If your last Self Assessment bill was over £1,000 and less than 80% of your tax was collected at source, HMRC asks for two advance payments towards the next year — each half of the previous year's bill, due 31 January and 31 July.
The first year this applies is the one that hurts: on 31 January you pay the year just ended in full, plus half of next year on top. Knowing that number in advance is exactly the kind of thing year-round records exist for.
What missing 31 January costs
A £100 penalty applies the moment the return is late, even if you owe no tax. Further penalties stack up at three, six, and twelve months, and late payment attracts interest from day one plus surcharges as time passes. None of it is negotiable after the fact; almost all of it is avoidable before it.
If you cannot pay in full, file anyway and set up a Time to Pay arrangement with HMRC — filing penalties and payment penalties are separate, and filing on time removes the first entirely.
How we run Self Assessment
Deadline Lock tracks every client's registration, filing, and payment dates, with advance notification before each one. Returns are prepared from records maintained year-round in the Evidence Pack — so January is a confirmation, not a scramble.
Frequently asked questions
When do I need to register for Self Assessment?
By 5 October following the end of the tax year in which you first had income to report. Start renting a property out in August 2026 — that is the 2026/27 tax year — and you must register by 5 October 2027.
Can I file after 31 January without a penalty?
No. The £100 penalty is automatic from 1 February, whether you owe tax or not. HMRC cancels it only for a reasonable excuse, which is a high bar — pressure of work and forgetting do not qualify.
Why is my January payment bigger than my tax bill?
Payments on account. If your bill was over £1,000, January includes the balance for last year plus 50% of that amount as an advance for the current year. July takes the second 50%. They are credits against next year's bill, not extra tax.
Can payments on account be reduced?
Yes, if you expect this year's income to be lower, you can apply to reduce them. Reduce them below what the final bill turns out to be, though, and HMRC charges interest on the shortfall — so the estimate needs to be honest, not hopeful.